Hypoxia in Atlantic salmon aquaculture: Current understanding and the way forward toward mitigation and predictive management

Hypoxia is an increasing challenge for Atlantic salmon aquaculture as climate-driven warming, stratification, harmful algal blooms, and changing coastal oxygen dynamics interact with high biomass production to reduce dissolved oxygen availability. This review synthesizes current knowledge on the causes, consequences, and management of hypoxia in Atlantic salmon farming, spanning environmental oxygen dynamics, physiological tolerance, production performance, welfare, and emerging mitigation strategies. We examine how low oxygen affects Atlantic salmon across biological scales, from respiratory and cardiovascular adjustments to metabolic reallocation, stress-axis activation, osmoregulatory disturbance, immune modulation, altered behavior, reduced appetite, impaired growth, disease susceptibility, mortality risk, and potential effects on product quality. Particular emphasis is placed on feed intake, aerobic scope, and life-stage vulnerability, including embryos, smolts, grow-out fish, and triploid salmon, as these determine both biological resilience and production outcomes. Approaches for mitigation include oxygenation and aeration technologies, improved cage and site design, real-time sensor networks, adaptive feeding strategies, functional feeds, selective breeding, hatchery conditioning, predictive models, and digital twins. Although technological capacity is advancing rapidly, key knowledge gaps remain in the mechanisms linking oxygen sensing to appetite, immunity, oxidative stress, epigenetic regulation, phenotypic plasticity, recovery after hypoxia, and heritable variation in tolerance. Long-term studies in large salmon under realistic multi-stressor farming conditions are especially needed. Integrating mechanistic biology with high-resolution monitoring and predictive decision-support tools will be essential to move hypoxia management from reactive mitigation toward proactive, welfare-oriented, and climate-resilient aquaculture.